1. Field of the Invention
This invention relates to a nitride semiconductor template and a light-emitting diode.
2. Description of the Related Art
Gallium nitride compound semiconductors, such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), and indium gallium nitride (InGaN) have attracted attention as light-emitting device materials capable of red through ultraviolet light emission. One growing method for these gallium nitride compound semiconductor crystals is a Hydride Vapor Phase Epitaxy (HVPE) growing method using metal chloride gas and ammonia as raw material.
A feature of the HVPE method is as follows. According to this method, it is possible to obtain a growth rate of 10 μm/hr to 100 μm/hr or higher which is remarkably higher than a typical growth rate of several μm/hr in other growing methods such as Metal Organic Vapor Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). For this reason, the HVPE method has been often used in the manufacture of a GaN free-standing substrate (see e.g. JP Patent No. 3886341) and an AlN free-standing substrate. Here, the term “free-standing substrate” refers to a substrate having such strength to hold its own shape and not to cause inconvenience in handling.
In addition, a light-emitting diode (LED) made of a nitride semiconductor is typically formed on a sapphire substrate. In its crystal growth, after a buffer layer is formed on a surface of the substrate, a thick GaN layer having a thickness of about 10 to 15 μm including an n-type layer is grown thereon, and an InGaN/GaN multiple quantum well light-emitting layer (several hundreds nm thick in total) and a p-type layer (200 to 500 nm thick) are grown thereon in this order. The GaN layer under the light-emitting layer is thick in order to improve the crystallinity of GaN on the sapphire substrate and the like. This is followed by electrode formation, resulting in a final device structure as shown in FIG. 3 which will be described later. In the case of growth with the MOVPE method, the crystal growth typically requires about 6 hours, and about half of 6 hours is the time required to grow a so-called “template portion” that are nitride semiconductor layer(s) e.g. GaN layer(s) under the light-emitting layer.
From the above, if it is possible to apply the HVPE method with the remarkably high growth rate to the growth of the template, it will be possible to substantially shorten the growth time, thereby dramatically reduce LED wafer manufacturing cost.
On the other hand, one example of technique for improving the light extraction efficiency by reducing the optical confinement in the semiconductor light-emitting device is disclosed by e.g. JP-A-2002-280611.
In JP-A-2002-280611, a first layer is surface-roughened to increase the light extraction efficiency to provide high brightness. Also, in JP-A-2002-280611, a substrate is surface-roughened to have the aforementioned effect. JP-A-2002-280611 uses a so-called PSS (Patterned Sapphire Substrate), so as to increase the light extraction efficiency to provide the high brightness.
In addition, since the template portion is a portion through which electric current flows laterally, the template portion is required to be low in resistance. It is because the driving voltage (forward voltage) of the LED will be high, if the resistance of the template portion is not low. That is, the template portion is an important part which has both a role to improve the crystallinity, thereby lessen crystal defects in an active layer and enhance the internal quantum efficiency, and a role to lower the forward voltage.